The present invention relates to a system for compensating the deterioration of an oxygen sensor in an emission control system for automotive engines.
An emission control system for an engine with a ZrO.sub.2 oxygen sensor (called O.sub.2 -sensor hereinafter) is widely used. The system comprises proportion and integration circuit means responsive to the output of the O.sub.2 -sensor for producing a PI signal, and a driver for operating a fuel supply device such as a fuel injector in accordance with the PI signal so as to close the air-fuel ratio to stoichiometry. The output of the O.sub.2 -sensor varies cyclically in accordance with the periodic change of the air-fuel ratio of the mixture supplied to the cylinders of the engine.
On the other hand, the O.sub.2 -sensor gradually deteriorates over a period of time due to chemical and physical action caused by high temperature and components of the exhaust gases. When the O.sub.2 -sensor deteriorates, it produces its output with delay. As a result, the air-fuel ratio deviates from stoichiometry.
Japanese Patent Laid Open No. 54-12044 discloses a system for compensating the deterioration of the O.sub.2 -sensor. The system is adapted to detect the drop of the maximum output voltage and to compensate the delay of the output of the O.sub.2 -sensor in accordance with the maximum voltage. However, since the drop of the output voltage occurs when the O.sub.2 -sensor greatly deteriorates, exhaust gas purification is not carried out for a long time before the compensation.